There should be no impedement that would make the passenger side any harder than the drivers side. I normally use a big "C" clamp to press in the calipers. Be sure there is no dirt or other material in the rubber boot around the piston (don't clean it out with anything that might rip the rubber boot). Make sure there is room for the excess fluid in the master cylindar and then press it in. It shouldn't be easy enough to press in by hand, you will almost certainly need some sort of mechanical force. I use the old disc pad still clipped into the cylindar as a surface to push on. This helps keep the cylindar lined up square to the caliper. You don't want to open the bleed screw.
The pressure bleeder works really well.